This invention relates to fused metal bond abrasive bodies and to methods of making the same.
It is known to use a metal matrix to hold superabrasive particles (e.g., diamond and cubic boron nitride) in an abrasive body. Such metal matrix abrasive bodies may be utilized in grinding wheels, such as pencil-edging wheels, and the like. Ideally, the bond between the metal matrix and the abrasive particles must be strong enough to retain the abrasive particles in the matrix as the abrasive particles abrade a workpiece.
It is also known that metal coatings may be used to improve the retention of abrasive particles in such metal matrices. For example, diamond abrasive particles may be advantageously coated with carbide forming metals which chemically bond to the surface of the diamond through the formation of a metal carbide. Metal coatings may add texture to the surface of diamond or cubic boron nitride abrasive particles which typically are smooth and difficult to bond to. A textured surface may allow the coated abrasive particle to be held more aggressively in the metal matrix through mechanical adhesion. Metals suitable for adhesion-promoting coatings include, for example, molybdenum, titanium, and chromium, which may be applied by a hot salt method or a vapor deposition method.
Typically, metal matrix abrasive bodies are formed by a fusing process. Fusing processes are well known and include, for example, sintering, brazing, melting, impregnation or combinations thereof. To form the metal matrix abrasive body, a fusible composition, typically comprising a metal powder and abrasive particles, is heated to a temperature for a period of time sufficient to consolidate the metal powder particles such that they bond to one another. Fusing by a sintering process, for example, is typically conducted in an air atmosphere at a relatively high temperature, for example, 700-1100 xc2x0 C., and at an elevated pressure. Under such conditions, oxidation of the various components of the sinterable composition may result. It has been recognized that oxidation of the very thin adhesion-promoting coating on the abrasive particles may deteriorate the adhesion-promoting function of the coating. Accordingly, materials and techniques have been developed to reduce or eliminate oxidation.
One method by which oxidation has been minimized is by coating an oxidation-resistant layer over the adhesion-promoting coating. This technique, however, adds expense to the abrasive particles since they must be coated with at least two materials. Further, the outer coating may not adhere well to the adhesion-promoting coating thereby resulting in a weak interface between the abrasive particle and the metal matrix. U.S. Pat. No. 5,024,860 reports the use of chromium, titanium or zirconium carbide-forming layer as part of a multi-layer coating on diamond particles to aid retention within a matrix. Two carbide-forming layers are applied; one thin base layer and a thick oxidation resistant second layer. The thick multi-layer construction provides increased oxidation resistance over thinner single coatings.
Oxidation may also be minimized by fusing (e.g., sintering) in a non-oxidizing atmosphere, for example, a nitrogen atmosphere or under very low air pressure. This type of process, however, is undesirable due to the high cost and process complexity associated with providing the non-oxidizing atmosphere. Specifically, fusing in a non-oxidizing atmosphere is typically conducted using an expensive vacuum furnace. In addition, if the fusible composition contains organic compounds (e.g., binders) that burn off during the fusing process, maintenance of the non-oxidizing atmosphere is further complicated. Another way in which oxidation may be minimized is by cleaning metal oxide contaminants from the metal powders prior to fusing the powders. This cleaning process adds an additional processing step and associated expense.
Although the foregoing techniques may be utilized to reduce oxidation of the adhesion promoting coating on the abrasive particles, what is desired is a more convenient method of reducing oxidation of adhesion promoting metal coatings on abrasive particles in fused abrasive bodies.
The present invention provides fused abrasive bodies comprising a plurality of metal coated abrasive particles bonded together by a metal matrix. The metal coated abrasive particles each comprise an abrasive particle having an outer adhesion-promoting metal coating. The fused abrasive body also comprises at least an effective amount of an oxygen scavenger metal. Suitable oxygen scavenger metals are selected to be competitively oxidized relative to the metal coating on the abrasive particles. In this way, oxygen present during the fusing of the abrasive body reacts, at least in part, with the oxygen scavenger metal thereby protecting the metal coated abrasive particles from oxidation. As a result, oxidation of the adhesion promoting coating on the abrasive particles is at least reduced, preferably eliminated. Suitable oxygen scavenger metals may be selected with the aid of an Ellingham diagram which predicts, at a given fusing temperature whether a given metal will be competitively oxidized relative to the metal comprising the adhesion promoting coating on the abrasive particles.
As used herein, the term xe2x80x9ccompetitively oxidizedxe2x80x9d means that the oxygen scavenger metal reacts with oxygen at a rate which is at least equal to, preferably greater than, the rate at which the metal comprising the adhesion promoting coating on the abrasive particles reacts with oxygen. More specifically, with reference to an Ellingham diagram a suitable oxygen scavenger metal (1) provides a partial pressure of oxygen at the fusing temperature which is less than or equal to the partial pressure of oxygen provided by the metal comprising the adhesion promoting coating on the abrasive particles at the fusing temperature; or (2) provides a Gibbs free energy of oxidation at the fusing temperature which is less than or equal to the Gibbs free energy of oxidation provided by the metal comprising the adhesion promoting coating on the abrasive particles at the fusing temperature.
Accordingly, in a preferred embodiment of the present invention the oxygen scavenger metal provides a partial pressure of oxygen at the fusing temperature which is less than or equal to the partial pressure of oxygen provided by the metal comprising the adhesion promoting coating on the abrasive particles at the fusing temperature.
In another preferred embodiment of the present invention the oxygen scavenger metal provides a Gibbs free energy of oxidation at the fusing temperature which is less than or equal to the Gibbs free energy of oxidation provided by the metal comprising the adhesion promoting coating on the abrasive particles at the fusing temperature.
In yet another preferred embodiment of the present invention, the abrasive particles comprise diamond, cubic boron nitride and the outer adhesion promoting coating on the abrasive particles comprises titanium, chromium, or an alloy thereof.
In yet another preferred embodiment of the present invention, the oxygen scavenger metal comprises aluminum, calcium, magnesium, zirconium or a combination thereof and is present in the fusible composition in an amount ranging from about 0.1-10% by-wt.
In fused abrasive bodies of the present invention the abrasive particles may be randomly or non-randomly distributed throughout the fused metal matrix. When non-randomly distributed, the abrasive particles may be concentrated in the fused metal matrix in substantially parallel planar layers of abrasive particles.
Metal matrix abrasive bodies of the present invention are particularly suited for use in cutting and grinding wheels. Accordingly, in yet another preferred embodiment of the present invention, a cutting or grinding wheel is provided comprising at least one metal matrix abrasive body of the present invention.
The present invention also provides a method of making a fused metal matrix abrasive body as described above, the method comprising the steps of:
(a) providing a fusible composition comprising:
a plurality of metal coated abrasive particles;
a bond metal powder; and
an effective amount of an oxygen scavenger metal
powder; and
(b) fusing the fusible composition of step (a) by sintering, brazing, melting or impregnating the fusible composition.
As used herein, the following terms have the following meanings:
xe2x80x9cFusedxe2x80x9d refers to a process wherein metal particles such as metal powders are bonded to one another by the application of heat. Fusing of metal particles may be accomplished by processes such as sintering, brazing, melting, impregnation or a combination thereof. Fusing of metal particles may be accomplished at temperatures above or below the melting point of the metal powders being fused and may include applying pressure to the fusible composition.
xe2x80x9cFusiblexe2x80x9d refers to a composition capable of being fused.
xe2x80x9cSinteringxe2x80x9d refers to the bonding of metal particles by solid-state reaction at temperatures lower than those required for the formation of a liquid phase. Fusible compositions of the present invention are typically sintered at temperatures ranging from about 700-1100xc2x0 C. and pressures ranging from about 100 to 500 kg/cm2.
xe2x80x9cBrazingxe2x80x9d refers to the process of bonding metal particles using a material having a melting point lower than the metal particles being joined.
xe2x80x9cMeltingxe2x80x9d refers to the process wherein metal particles are bonded to one another by converting the metal particles from a solid to a liquid by application of heat.
xe2x80x9cImpregnationxe2x80x9d refers to the process of forcing a liquid substance into the pores of a solid.
In a preferred embodiment of the present invention, the oxygen scavenger metal is added to the fusible composition in form of a substantially pure metal. By substantially pure it is meant that the oxygen scavenger metal is added to the fusible composition in a form comprising at least 50%-wt or greater oxygen scavenger metal, more preferably 80%-wt. or greater oxygen scavenger metal, yet more preferably 95%-wt or greater oxygen scavenger metal and most preferably 99%-wt. or greater oxygen scavenger metal.
In another preferred embodiment of the present invention, the fusible composition further includes a binder, most preferably a polymeric material such as polyvinyl butyral.
In yet another preferred embodiment of the present invention, the bond metal powder and the oxygen scavenger metal powder are provided in the form of a bond material layer in the form of a sheet having a first major surface and a second major surface. Prior to fusing, the abrasive particles are distributed over at least one major surface of the bond material layer to form the fusible composition. The abrasive particles may be distributed over the major surface of the bond material layer in a non random array.
In yet another preferred embodiment of the present invention, the fusible composition is prepared according to the following method:
(a) providing a bond material layer in the form of a sheet, the bond material layer comprising a metal powder, an effective amount of an oxygen scavenger metal and a binder;
(b) providing a porous sheet material having a first major surface a second major surface and a plurality of openings extending from said first major surface to said second major surface;
(c) adhering an adhesive tape to one major surface of the porous sheet material;
(d) positioning metal coated abrasive particles in at least some of the openings of the porous material to form an assembly; and
(e) placing the assembly of step (d) in contact with at least one major surface of the bond material layer of step (a) to form a fusible composition.
In a preferred embodiment of this method, a fusible composition including more than one substantially parallel planar layers of metal coated abrasive particles is prepared by stacking 2 to 10,000 of the fusible compositions of step (e) one on top of another. The fusible composition is then fused to provide an abrasive body comprising more than one substantially parallel planar layers of metal coated abrasive particles.